public class Solution4 {
    public int m;
    public int n;
    public int[] dx = { 0, 0, 1, -1};
    public int[] dy = { 1, -1, 0, 0};

    public void solve(char[][] board) {
        m = board.length;
        n = board[0].length;

        // 1. 先把边界的 O 相连的连通块，全部修改成 '.'
        for(int j = 0; j < n; j++) {
            if(board[0][j] == 'O') {
                dfs(board, 0, j);
            }
            if(board[m - 1][j] == 'O') {
                dfs(board, m -1, j);
            }
        }

        for(int i = 0; i < m; i++) {
            if(board[i][0] == 'O') {
                dfs(board, i, 0);
            }
            if(board[i][n - 1] == 'O') {
                dfs(board, i, n - 1);
            }
        }

        // 2. 还原
        for(int i = 0; i < m; i++) {
            for(int j = 0; j < n; j++) {
                if(board[i][j] == 'O') {
                    board[i][j] = 'X';
                }else if(board[i][j] == '.') {
                    board[i][j] = 'O';
                }
            }
        }
    }

    public void dfs(char[][] board, int i, int j) {
        board[i][j] = '.';
        for(int k = 0; k < 4; k++) {
            int x = i + dx[k];
            int y = j + dy[k];
            if(x >= 0 && x < m && y >= 0 && y < n && board[x][y] == 'O') {
                dfs(board, x, y);
            }
        }
    }
}
